As for Onana, I believe he is a solid keeper but he is under a lot of pressure right now to deliver and the more mistakes he makes, the more pressure he gets and the more he fumbles. It's like a downward spiral for him right now and except Ten hag does something to fix his defensive team including Onana, they'll continue to suffer from defensive mistakes and errors.
He’s a great goalie and you’re right. He was brought in to the squad with a lot of expectations from fans and management hence why he’s probably under a lot of pressure right now. Man United’s defense too isn’t making things better for him also as they share some blame in all this.
With more pressure mounting on him to protect the goal at all costs, more mistakes are bound to happen and the sooner Ten Hag fixes these problems, they’ll have to deal with loosing more games.
Onana has owned up to his mistakes in the game and hopefully, he does learn from it.